Comparing nutrients and volatiles among different tomato varieties

Background: Flavor is an important quality of tomato fruit. The improvement of flavor attracts more and more attention. This study aimed to explore the differences in the concentrations of nutrients and volatiles between red and pink colors of tomatoes fruit, including cherry tomato ( S. lycopersicum var. cerasiforme Mill) and large-fruited tomato ( S. lycopersicum ), respectively. Methods: Soluble sugar, titratable acids, and volatile organic compounds, were detected using gas chromatography-mass spectrometry. Hedonism score and odor activity value were used to evaluate the taste and odor intensity of tomato fruit. The membership function method was used to comprehensively evaluate the fruit flavor. Results: It was found that the levels of aldehydes, ketones, esters, and phenols were significantly higher in pink tomato than in red tomato. The concentrations of ascorbic acid, soluble solids, fructose, glucose, citric acid, and carotenoid-derived volatiles were significantly greater in cherry tomato than in large-fruited tomato. However, Phe-derived and Ile/Leu-derived volatiles were significantly higher in pink large-fruited tomato and red cherry tomato. The fatty and irritant odors were stronger in pink tomato than in red tomato, and cherry tomato had better overall taste than large-fruited tomato. The sweetness and sweetness/acidity ratio were significantly higher in pink cherry tomato than other categories of tomatoes. Conclusion: The concentrations of volatiles varied greatly between pink and red tomatoes, and the levels of nutrients varied greatly between cherry and large-fruited tomatoes. This study can provide reference for tomato flavor quality improvement breeding.


Flavor sensory evaluations of red and pink tomato
The scores of the sensory evaluation are shown in table 3 and Additional file 1. All of the sensory evaluation scores were higher in pink tomato than those in red tomato. Pink cherry tomato had significantly higher sweetness (6.31 points) and sweetness/acidity ratio than other categories of tomatoes. Besides, the sweetness, overall taste, tomato characteristic flavor, sweetness/acidity ratio, and were more 2.09, 1.52, 1.01, and 0.6 points higher in pink cherry tomato than red cherry tomato, respectively. Correspondingly, their scores were higher 0.45, 1.15, 0.5, and 0.12 points in pink largefruited tomato than in red large-fruited tomato. Whereas the acidity score was more 0.39 point in red cherry tomato than in pink cherry tomato.

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Comprehensive flavor evaluation values can show that pink tomato have better flavor than red tomato, and cherry tomato are more delicious than large-fruited tomato.

Correlation analyses of key flavor factors in tomato fruit
In order to further explore the contribution of nutrients and volatiles to tomato flavor, the pearson correlations were studied between compound concentration and flavor intensity. For the sake of brevity, only the items are shown in Table 4 and Additional file 1 with a significant correlation or very significant correlation.

Discussion
Fruit color is an important commodity character [16], which will affect consumers' purchasing desire.
Pink and red are the most common tomato fruit colors. Pink tomato is caused by a mutation in the SlMYB12 of red tomato, resulting in a lack of yellow-colored flavonoid naringenin chalcone in the peel [17]. Pink tomato took a large market proportion in Asia due to their fruit with bright color, uniform size, fine flesh, and strong flavor [8]. Lycopene, β-carotene concentrations and taste index were higher in pink tomato than those in red tomato [18]. Carotenoids had diverse bioactive and chemical properties [19]. Pink tomato had higher nutrition and health care effect. This study found that pink tomato had higher soluble solids, fructose, glucose, aldehydes and ketones than red tomato, which made pink tomato have higher sweetness, tomato characteristic flavor, and overall taste.
Fruit size is one of traits affecting consumer preferences. Both the smaller and bigger size tomatoes are tastier comparing with medium size tomatoes [18]. Cherry tomato is popular among consumers with high sweetness and fine fruit type. In this study, cherry tomato had a significantly higher nutrients and volatiles; namely, ascorbic acid, soluble solids, fructose, glucose, citric acid, alcohols, aldehydes, ketones, and esters. More concentrations of flavor compounds make the flavor better.
Comprehensive flavor evaluation of cherry tomato was significantly higher than large-fruited tomato.
Because cherry tomato had small fruit size, which made higher concentrations of photosynthetic products, sweetness, etc. As table fruit, cherry tomato is more and more popular with consumers.
Previous study said that cherry tomato had higher fructose, glucose, soluble solids, total polyphenols, carotenoids, sweetness, and acidity than fresh market tomatoes, which were good for consumers' health [35,36].
Since the same category of tomatoes were compared with other categories of tomatoes as a whole, some tomato varieties were not reflected with special flavor characteristics, e.g. the rankings of comprehensive flavor evaluation of two red cherry tomato, CI1009 and CI1005 varieties, were the second and third among 76 tomatoes varieties. To our surprise, one red large-fruit tomato, TI1022 variety, ranked the sixth in flavor. 9

Conclusions
The flavor was better in pink tomato than in red tomato, while the flavor was better in cherry tomato than in large-fruited tomato. Pink tomato had higher volatiles levels with better odor than red tomato.
Cherry tomato had more nutrient contents with better taste than large-fruited tomato.
There was a strong correlation between nutrients concentrations and sensory evaluation scores, the same to volatiles levels and odors intensity. However, there was lower correlation, between volatiles levels and sensory evaluation scores or between nutrients contents and odors intensity.

Tomato varieties
The tomato varieties for the experiment were shown in Additional file 3, including 8 pink cherry Where A is the absorbance at OD 760 nm . c is the AsA concentration (kg L -1 ) in the sample obtained from the standard curve, V is Total volume of the sample extract (L), W is the mass of tomato fresh tissue (kg), and a is the volume of the sample extract (L) used in the measurement.
c. The soluble solids were measured by dripping tomato juice directly on a PAL -1 digital refractometer (Japan Atago Co., Ltd.).
d. Soluble sugar and titratable acids were determined using gas chromatography-mass spectrometry (GC / MS) method. The methoxyamine was used to derivatize treatment [39,40].
e. Qualitative and quantitative analysis of volatile aromatic compounds was taken by headspace solidphase microextraction (HS-SPME) gas chromatography-mass spectrometry (GC/MS) method [41][42][43]. RT of each normal alkane was measured under the same GC/MS conditions after mixing the C 4 -C 26 normal alkanes standard solution. Then, RI was calculated of each volatile compound according to the following formula: t'R is the retention time, and Z and Z + 1 are the number of carbon atoms in the normal paraffin before and after the target volatiles (X) flow out, respectively. t'R (z) <t'R (x) <t'R (z + 1).
Qualitative analysis of volatiles was compared with the standard mass spectrum of the library (NIST2011, USA) and RI [44]. Only the volatiles were selected, whose both positive and negative matches were greater than 800 with mass spectrometry. The peak area normalization method was used to calculate the relative concentration of various volatiles. Due to errors were inevitable in GC/MS measurement values. The measured volatiles concentrations must be corrected according to 11 the calibration curve. In this study, 0, 5 × 10 -6 , 10 -5 , 1.5 × 10 -5 , 2 × 10 -5 , 2.5 × 10 -5 , or 3 × 10 -5 L 3nonanone standard solutions (3.284 × 10 -5 kg L -1 ) was added respectively to 0.005 kg different tomato samples. 3-Nonanone concentration was measured under the same GC/MS conditions in different samples. Linear regression was performed between the concentration measured by GC/MS and the theoretically calculated concentration. The calibration curve was as follows: y = 1.004x-0.044, R 2 = 0.998 (4) According to the calibration curve (4), the concentration of each volatile was determined.
f. Sensory evaluation methods of sweetness, acidity, characteristic flavor and overall taste referenced literature [2,38,[44][45][46][47], slightly changed. Different tomato samples were numbered and cut to form wedge shapes. The panels of trained and tasting wore eye patches to evaluate tomato flavors. The panels are made up of 25 males and 25 females, whose ages are between 18 and 60 years old. In advance, we tentatively had a maximum score of 8.00 points. They scored the sweetness, acidity, tomato characteristic flavor and overall taste according to the flavor intensity and sensory pleasure.
The stronger the flavor, the higher the score. The better the sensory pleasure, the higher the score.
After tasting a sample, the panels should rinse the mouth with purified water three times. To reduce taste and smell fatigue, tasted for 2700 s and rested for 900 s.

Data statistics
All test data was recorded by WPS Office 2019. The tomato varieties were divided into four categories for pink cherry tomato, pink large-fruited tomato, red cherry tomato, and red large-fruited tomato varieties. Data for each measure were averaged in the same category of tomato varieties. The intragroup SD and the variation coefficient of the same category tomato varieties were calculated. Then, the date differences were analyzed among the four categories by one-way analysis of variance (P < 0.05) using SPSS 17.0. Z-Score standardized the data of tomato nutrients and volatiles levels and sensory evaluation scores. Pearson correlation analysis was performed among different flavor factors.
Then, the membership function method was used to evaluate the flavor of each tomato variety comprehensively. The principal component analysis was performed (suppose two principal components were extracted). Comprehensive flavor evaluation value (Y) of each tomato variety 12 calculated according to the following formula: Y = (α 1 × F 1 + α 2 × F 2 )/(α 1 + α 2 ) (5) α 1 means the variance contribution rate of principal component 1 (PC1

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Not applicable.

Consent for publication
Not applicable.

Availability of data and materials
All data generated or analyzed during this study are included in this published article and its supplementary information files.

Competing interests
The authors declare that they have no competing interests.

Supplementary Information
Additional file 1.xls: Table S1. The concentrations of nutrients (%) and volatiles (10 -9 kg L -1 ) in each tomato accession and their sensory evaluation.